A new air-and-ground search was launched today for the 26-year-old hiker who disappeared in a Southern California desert on Sunday while camping with his family.

Guillermo Pino could have fallen into a mud cave or down a ravine in the "challenging" terrain of Anza-Borrego Desert State Park, according to the San Diego County Sheriff's Department. The park, which is the largest state park in California, covers some 600,000 acres and is filled with mud caves, steep dirt trails and ravines.

Pino was last seen Sunday, around noon, while camping in the park with his family, Sheriff's Department spokeswoman Jan Caldwell said today. Pino was reportedly standing atop a cliff above where his family was standing, and asked them to climb up to him to bring him a pair of shoes. Pino had been exploring a network of dirt tunnels and caves that run through hillsides in the area, and had taken off his shoes and head lamp, according to ABC News affiliate KGTV. The family hiked up to meet Pino with his shoes, but when they arrived, he was not there.

"He was up there just enjoying Easter morning looking at the vista," said Caldwell. "We don't know if he fell into an open part of a cave and is trapped underground or somehow wandered off or if he fell or started exploring a cave and became disoriented."

Caldwell said that because of temperate conditions within the park, there is a chance that Pino could still be alive. Pino's family described him as a fairly experienced hiker with no known medical conditions.

Pino may have been able to find a small amount of drinking water collected in the caves, she said, although there is not much water in the park as a whole.

Helicopters, canines and dozens of volunteers combed the park for a fourth day today, searching the "very difficult" terrain in the park. The area of the park where Pino disappeared is covered in mud caves and ravines, and is colloquially called the "Badlands," according to the sheriff's department.

"The topography is made up of an expansive honeycomb of caves," Caldwell said.

Surrounding counties and the U.S. Border Patrol have lent resources to the search for Pino. He is described as 6 feet tall, 180 pounds, wearing jeans and a red shirt with no shoes. Pino did not have any food or water, nor did he have a flashlight or a cell phone with him when he disappeared, the department said.

Family members and friends of Pino said it was his first time in the park, and he was enjoying the trip.

"We have been there lots of times before, but it was Guillermo's first time," Michelle Pino-Knaak, Pino's sister, told KGTV. "My husband and another walked all over that mountain retracing our steps. We saw his footprints but we couldn't find him."

Hajira Kahn, a member of the group, told KGTV, "We left the cave and saw him at the top, and he said, 'It's so beautiful up here.'"

Pino's sister, reached at the family's home, declined to comment. Other family members are continuing to search today.